Time for a quiet, loving heart to heart conversation with your daughter. Topics to discuss:
She's picking up extra shifts for money. Is this for weekly spending money? For an emergency fund? For an upcoming big purchase or settlement of a bill/credit card? What total amount is she trying to save.
When will all this overtime end? How long will it take for her to save this amount of money.
You're older and work full time. You simply do not have the energy to do full time caretaking on weekends, and it's starting to affect your health. This can't continue. Period.
She needs to look into other weekend care. How about her siblings or those other bitching relatives? It's eight (weekend)days each month, I'm sure she can figure out a rotating schedule that doesn't burden anyone too much.
It's time for you to tell her that she must make other arrangements. This is not healthy for you and is unsustainable.